Since a lithium ion secondary battery has a low weight and excellent input and output characteristics in comparison with a nickel-cadmium battery, a nickel-hydrogen battery, and a lead battery, which are secondary batteries in the related art, the lithium ion secondary battery is recently anticipated as a power source for electric cars or hybrid cars. In general, such a type of battery has a structure in which a positive electrode (cathode) including lithium which can be reversibly intercalated and a negative electrode (anode) formed of a carbon material face each other with a non-aqueous electrolyte interposed therebetween. Therefore, these batteries do not enter a dischargeable state when they are not assembled in a discharged state and then charged. Hereinafter, charging and discharging reactions will be described with reference to an example in which a positive electrode including lithium cobalt oxide (LiCoO2), a negative electrode including a carbon material, and a non-aqueous electrolyte solution including a lithium salt as an electrolyte are used.
First, when a first charging cycle is performed, lithium included in the positive electrode is emitted to the electrolyte (Formula 1) and the positive electrode potential goes in the higher direction. In the negative electrode, lithium emitted from the positive electrode is occluded in the carbon material (Formula 2), and the negative electrode potential goes in the lower direction. In general, when the difference between the positive electrode potential and the negative electrode potential, that is, the battery voltage, reaches a predetermined value, the charging reaction is cut off. This value is called charging cutoff voltage. In the discharging reaction, lithium occluded in the negative electrode is emitted, the negative electrode potential goes in the higher direction, lithium is occluded in the positive electrode again, and the positive electrode potential goes in the lower direction. Similarly to the charging reaction, when the difference between the positive electrode potential and the negative electrode potential, that is, the battery voltage, reaches a predetermined value, the discharging reaction is cut off. This value is called discharging cutoff voltage. The total charging and discharging reaction formula is expressed by Formula 3. In the second cycle or the subsequent cycles thereto, lithium comes and goes between the positive electrode and the negative electrode to progress the charging and discharging reaction (cycle).

The carbon material used as the material for a negative electrode of a lithium ion secondary battery is generally approximately classified into a graphite-based carbon material and an amorphous carbon material. The graphite-based carbon material has an advantage that the energy density per unit volume is higher than that of the amorphous carbon material. Therefore, the graphite-based carbon material is generally used as the material of a negative electrode in a lithium ion secondary battery for mobile phones or notebook computers requiring a compact structure and a large charging and discharging capacity. Graphite has a structure in which reticulated plane of carbon atoms are regularly laminated, and intercalation and deintercalation reactions of lithium ions progress in an edge portion of crystallites during charging and discharging.
As described above, such a type of battery is actively studied as a power storage device for vehicles, industry, and power supply infrastructure. When the batteries are used for these applications, very high reliability is required, compared with a case in which they are used for mobile phones or notebook computers. Here, reliability is a characteristic associated with the service life and can be said to be a storage characteristic. That is, reliability means a characteristic that charging and discharging capacity or internal resistance are not changed much (not degraded much) even when the charging and discharging cycles are repeated, even when the battery is stored in a charged state with a predetermined voltage, or even when the battery continues to be charged with a constant voltage (when floating-charged).
On the other hand, it is generally known that the service life characteristics of lithium ion secondary batteries having been used for mobile phones or notebook computers in the related art greatly depend on the material of the negative electrode. This is because it is not possible in principle to set the charging and discharging efficiencies of the positive electrode reaction (Formula 1) and the negative electrode reaction (Formula 2) to be completely equal to each other and the charging and discharging efficiency of the negative electrode is lower. Here, the charging and discharging efficiency means the ratio of dischargeable electric capacity to the electric capacity consumed in charging. Hereinafter, the reaction mechanism in which the service life characteristic degrades due to the low charging and discharging efficiency of the negative electrode reaction will be described in detail.
In the process of charging, as described above, lithium in the positive electrode is emitted (Formula 1) and is occluded in the negative electrode (Formula 2), but the electric capacity consumed in the charging is the same in the positive electrode reaction and the negative electrode reaction. However, since the charging and discharging efficiency is lower in the negative electrode, the discharging may be cut off in a state in which the amount of lithium emitted from the negative electrode is smaller than the amount of lithium which can be occluded in the positive electrode, that is, the amount of lithium occluded in the positive electrode before the charging, in the subsequent discharging reaction. This is because a part of the electric capacity consumed in the charging in the negative electrode is consumed in side reactions and competitive reactions but is not consumed in the reaction in which lithium is occluded, that is, the reaction in which lithium is occluded as the dischargeable capacity.
Since such charging and discharging reactions occur, the positive electrode potential in the discharging cutoff state goes to a side higher than the original potential before the charging and discharging reactions, and the negative electrode potential also goes to a side higher than the original potential before the charging and discharging reactions. This is because all lithium emitted in the charging process of the positive electrode is not occluded (is not returned) during the discharging, the potential has gone in the higher direction in the charging process cannot be returned to the original positive electrode potential by the amount corresponding to the difference in the charging and discharging efficiency between the positive electrode and the negative electrode even when it goes in the lower direction in the discharging process, and thus the discharging is cutoff at a potential higher than the original positive electrode potential. As described above, the discharging of a lithium ion secondary battery is completed when the battery voltage (that is, the difference between the positive electrode potential and the negative electrode potential) reaches a predetermined value (the discharging cutoff voltage). Accordingly, when the potential of the positive electrode at the time point of cutting off the discharging becomes higher, the potential of the negative electrode also goes in the higher direction by the same amount.
As described above, when such a type of battery repeats the charging and discharging cycles, there is a problem in that the capacity obtained within a predetermined voltage range (within the range of the discharging cutoff voltage and the charging cutoff voltage) degrades due to the change in operational range of the capacity of the positive and negative electrodes. The reaction mechanism of such a capacity degradation has been reported in academic societies (Non-Patent documents 1 and 2). The positive electrode potential and the negative electrode potential of which the operational ranges once changed are irreversible and cannot be returned to the original state, and the fact that there is no means for recovering the capacity makes this problem severe.
The reaction mechanism of capacity degradation occurring when the charging and discharging cycles are repeated is basically the same as the reaction mechanism of capacity degradation when a battery is stored in a charged state or the reaction mechanism of capacity degradation when the floating-charging is performed. First, when a battery is stored in a charged state, it is known that the capacity consumed in side reactions and competitive reactions occurring in the charged state, that is, the self-discharged capacity, is larger in the negative electrode than in the positive electrode and thus the operational ranges of capacity of the positive and negative electrodes are changed before and after storage, thereby causing the battery capacity after storage to degrade (Non-Patent document 3). Similarly to the difference in charging and discharging efficiency between the positive electrode and the negative electrode, the difference in self-discharging rate between the positive electrode and the negative electrode in the charged state results from the fact that the rates of the side reactions and the competitive reactions occurring in the negative electrode in the charged state are similarly higher than the rates of the side reactions and the competitive reactions occurring in the positive electrode in the charged state.
When floating-charging is performed, the positive electrode potential and the negative electrode potential continue to be charged with predetermined potentials in the initial charging period. However, the current value (the leak current in the positive electrode) necessary for maintaining the positive electrode potential at the potential and the current value (the leak current in the negative electrode) necessary for maintaining the negative electrode potential at the potential are actually different from each other. Therefore, when the floating charging is performed, the leak current in the negative electrode is greater than the leak current in the positive electrode, and thus the negative electrode potential migrates in a direction in which the leak current decreases, that is, in the higher direction, and the positive electrode potential goes in a direction in which the leak current increases, that is in the higher direction. In this way, when the floating charging is performed, there is a problem in that the operational ranges of capacity of the positive electrode and the negative electrode are irreversibly changed and the battery capacity degrades.
A “raw coke composition obtained by coking a heavy oil composition through the use of a delayed coking process” is generally known as a raw material of the negative electrode material of a lithium ion secondary battery. The delayed coking process is very suitable for mass-producing high-quality carbon materials, and various types of coke mass-products are produced through the use of this process. Graphite materials obtained through graphitization such that the crystallite size Lc(112) of the (112) diffraction line measured through the use of a wide-angle X-ray diffraction method is 4 nm or more are generally used as the graphite material of the negative electrode of the lithium ion secondary batteries (for example, see Patent document 1).
A graphite material obtained by pulverizing and classifying a raw coke composition so as to have a predetermined particle size distribution and performing a mechanochemical process on the product to cause crystals to highly develop at a temperature of 2800° C. or higher under an atmosphere of inert gas and a manufacturing method thereof are generally known (for example, see Patent document 2).
In Patent document 2, it is mentioned that the crystalline structure of the particle surface layer can be disturbed by pulverizing and classifying a raw coke composition and then performing the mechanochemical process (see Paragraph [0024] of Patent document 2). Since this disturbance of the crystalline structure remains as unorganized carbon even after the graphitization as the final process, the initial charging and discharging efficiency of the negative electrode can be improved (see Paragraph [0024] of Patent document 2), but there is a problem in that the reliability of the battery cannot be improved.
It has been reported that the crystalline structure of the surface of the graphite particles can be disturbed by applying a compressive shearing stress thereon using scale-like natural graphite as the negative electrode material of the battery (for example, see Non-Patent document 4). It has been reported that the disturbance of the crystalline structure of the graphite particle surface can improve the initial charging and discharging efficiency of the negative electrode (Paragraph of Patent document 2).
However, when mechanical energy based on a compressive shearing stress is applied to the graphite particles, the compressive shearing stress is applied to the particle surface to cut carbon-carbon bonds around the particle surface and to expose edge portions from the cut portions. Accordingly, it is thought that the graphite material in which many edge portions are exposed from the particle surfaces is obtained. In a lithium ion secondary battery using the graphite material as the negative electrode material, since the electrolyte in the edge portions exposed from the particle surfaces of the negative electrode is decomposed to increase the leak current in the negative electrode and to increase the difference from the leak current in the positive electrode, there is a problem in that the capacity retention rate (storage characteristics) is greatly lowered when the battery is held at high temperatures or normal temperatures for a long time.
In lithium ion secondary batteries manufactured using a graphite negative electrode with a highly crystalline structure, high electric capacity is obtained. However, when such a graphite material is used, it is said that a phenomenon in which an electrolyte is co-intercalated between graphite layers formed of reticulated plane with a high degree of parallelism from the edge portions of the crystallites and is then decomposed easily occurs at the same time as inserting lithium into the graphite crystals (Solvent Co-Intercalation Model of Besenhard, see Non-Patent document 5). The charging and discharging efficiency of the negative electrode is lowered by the side reactions and the competitive reactions due to the decomposition of the electrolyte between the graphite layers, thereby causing capacity degradation. It is also said that as the graphite crystals develop further, the solvent co-intercalation occurs more easily. Therefore, a technique of introducing disturbance of a crystalline structure into the particle surfaces through the use of the solvent co-intercalation so as to suppress the decomposition of the electrolyte has been reported. In Patent document 1, it is mentioned that the highly crystalline structure on the particle surfaces can be disturbed by pulverizing and classifying a raw coke composition and then performing the mechanochemical process. Since the introduced disturbance of the crystalline structure remains as an area having a low crystallinity even after the graphitization as the final process, it is mentioned that it is possible to improve the initial charging and discharging efficiency of the negative electrode (Paragraph [0024] of Patent document 2). However, it is thought that the disturbance of the crystalline structure introduced through the mechanochemical process is a so-called isotropic state in which crystallites of unorganized carbon are randomly oriented and many edge portions are exposed from the particle surfaces.
In general, in the edge portions of crystallites, plural dangling bonds, that is, many localized electrons of which valence electron bonds are not saturated and which are present without bonding opponents, are present. On the surface of the carbon material of the negative electrode, that is, in the interface between the electrolyte and the carbon material, in the charging process, it is thought that side reactions and competitive reactions due to the reductive decomposition of the electrolyte based on the catalytic action of the localized electrons occur in addition to the original charging reaction in which lithium is inserted into the graphite crystals, thereby lowering the charging and discharging efficiency of the negative electrode. That is, even when an area having a low crystallinity is introduced into the particle surfaces to suppress the decomposition of the electrolyte due to the solvent co-intercalation, the crystallites in the introduced areas having a low crystallinity are in the isotropic state and thus the edge portions are exposed from the surface to increase the reductive decomposition of the electrolyte, thereby causing the capacity degradation.
The inventors had an idea that the charging and discharging efficiency of the negative electrode can be improved and the storage characteristics of a lithium ion secondary battery can be improved, by employing a graphite material having a structure in which areas having a low crystallinity are introduced into the highly crystalline structure and having small exposure of crystallite edges from particle surfaces, and actively studied to reach the present invention.